   can be slow. So, you won't want this interceptor present in production
   code. So condition it out like this :
 
-    (re-frame.core/reg-event-db
-       :evt-id
-       [(when ^boolean goog.DEBUG re-frame.core/debug)]  ;; <-- conditional
-       (fn [db v]
-         ...))
+      (re-frame.core/reg-event-db
+         :evt-id
+         [(when ^boolean goog.DEBUG re-frame.core/debug)]  ;; <-- conditional
+         (fn [db v]
+           ...))
 
   To make this code fragment work, you'll also have to set goog.DEBUG to
   false in your production builds - look in `project.clj` of /examples/todomvc.


  These handlers take two arguments;  `coeffects` and `event`, and they return `effects`.

  (fn [coeffects event]
     {:db ...
      :dispatch ...})

   Wrap handler in an interceptor so it can be added to (the RHS) of a chain:
     1. extracts `:coeffects`


  Usage:

      (defn my-f
        [a-val b-val]
        ... some computation on a and b in here)

      (on-changes my-f [:c]  [:a] [:b])

  Put this Interceptor on the right handlers (ones which might change :a or :b).
  It will:
